Just peel them, chop into small segments and microwave for about 7-8 minutes in a bowl with a good sprinkle of Demerara sugar and cover with cling film. Stir after 7 minutes and keep microwaving until they're soft but not too mushy. They'll cook again in the crumble later.
For the topping I use 100g plain flour to 75g butter, 50g rolled oats and 100g Demerara sugar. You can scale it up depending on the size of your dish and how much fruit you've got. For the size below I doubled it. Then add your topping and bake for 50 minutes until bubbling. It's even better if you bake it then warm it up again later or the next day. Yum!